A lead frame is used as the electrical connection between a semiconductor chip and the printed circuit board (and thus to other electrical components). Lead frames are typically constructed of a base metal (e.g. copper) onto which subsequent metal layers may be deposited to enhance properties such as solderability. It is increasingly popular to plate layers of nickel, palladium and gold in order to get good adhesion of lead free solder to the lead frame surface. Lead frames are usually manufactured from a continuous strip of copper or copper metal alloy (optionally plated with additional layers) onto which a pattern is repeatedly stamped or etched comprising a central die pad that multiple inner leads extend out from to outer leads, which form the connection of the package to the board. Then, an adhesive is dispensed onto the die pad and a semiconductor chip called a die is placed on top and the adhesive is cured. Electrical connections are then made between the top of the semiconductor die and the leads via ultrasonically welded thin gold wires. This assembly is quite fragile, so it is protected by encapsulating it in an epoxy molding compound that provides mechanical durability to the assembly. After curing, the assembly is sectioned from the adjacent packages and it is connected to a printed circuit board (PCB) via soldering the leadfingers extending from the assembly to pads on the PCB. A typical semiconductor device is shown in FIG. 1.
An example of another semiconductor device is a “QFN” (Quad Flat Pack No Lead). Such a configuration is shown in FIG. 2 in which the leads are located on the bottom of the semiconductor device and are exposed as shown in FIG. 2 for soldering to the circuit board.
The lead frame is made from a conductive metal typically copper, a copper alloy, iron, or an iron alloy. Copper is preferred because of its corrosion resistance, electrical conductivity and solderability. The lead frame can also be pre plated, so named because the lead frames are plated prior to semiconductor device assembly. For example, a pre plated lead frame typically comprises a copper base that is electroplated (partially or fully) with a layer of nickel and then a thin layer of palladium followed by a flash layer of gold. FIG. 7 shows a schematic cross-section of a pre plated lead frame. The pre plated lead frames are desirable because they allow the use of environmentally friendly lead-free solders to attach the leads to the circuit board. Also, copper leads such as shown in FIG. 1 must be presoldered with lead-tin solders before attachment to the circuit board. This often results in “tin whiskers” contacting adjacent leads resulting in short circuiting of the semiconductor device. Pre plated leads such as those described above do not require presoldering and avoid the tin whisker problem.
After plating, an electrically and thermally conductive adhesive (called a ‘die attach adhesive’) is dispensed onto the central die pad then a die is placed on top of the adhesive layer. This assembly is then cured to fix the die to the die pad, providing a conductive path between the two. During this process, a common problem that occurs is called ‘epoxy bleed out’ where some of the organic vehicle (epoxy and reactive diluents, for example) bleed out of the adhesive and spread across the lead frame surfaces. This bled out layer of organics can have drastically negative effects on other processes and materials, such as wire bondability, solderability and mold compound adhesion. Reduction in these properties typically results in a poorer package that is more susceptible to environmental stresses and is overall less reliable. Due to this problem of epoxy bleed there is a pressing need in the industry to develop materials or methods to limit or stop this phenomenon. After the die attach step, the semiconductor die is then connected to the leadfingers by ultrasonically welding gold wires from pads on the die top to the leadfingers. This is then followed by encapsulation of the entire assembly in an epoxy molding compound.
Subsequent to encapsulation, the outer leads of the lead frame are soldered to a circuit board. During soldering, the temperatures of the encapsulated package may rise from about 200° C. to about 260° C. Particularly susceptible to this temperature increase are the QFN semiconductor devices. The rapid increase in temperature and subsequent cooling stresses the adhesive bond between the lead frame and the encapsulating plastic often resulting in failure along the plastic/metal interface. This may lead to moisture entering the assembly and subsequent failure of the semiconductive device.
To minimize separation between the encapsulating plastic and the metallic lead frame, several means to improve the adhesion have been proposed. These solutions include both means to increase mechanical adhesion and chemical adhesion. To improve mechanical adhesion, various configurations of holes, grooves and hemispheres have been formed in both the leads and the die pad. The holes and deformations increase the surface area of the lead frame component and also provide crevices for enhanced mechanical locking. For example, U.S. Pat. No. 4,862,246 to Masuda et al. discloses forming a series of hemispherical depressions on the die pad. These depressions increase the adhesion of the die pad to the molding resin increasing resistance to humidity.
A layer of nickel applied to a copper alloy lead frame has been found to increase the strength of the metal/plastic bond as disclosed in U.S. Pat. No. 4,888,449 to Crane et al. U.S. Pat. No. 4,707,724 to Suzuki et al. discloses coating the die pad with an alloy of tin/nickel or iron/nickel to increase adhesive strength.
Certain chemical solutions also increase the adhesive strength of the bond between copper and a plastic. U.S. Pat. No. 4,428,987 to Bell et al. discloses pretreating the copper surface to improve adhesion. The surface is electrolytically reduced and then coated with a solution such as benzotriazole. U.S. Pat. No. 5,122,858 discloses coating the lead frame with a polymer coating such as a polyolefin or a polyimide. U.S. Pat. No. 7,329,617 also discloses coating the lead frame with a coating based on a nitrogen-containing polymer such as a melamine-functional phenolic resin.
While the prior art processes are somewhat effective to increase the adhesion between the molding resin and the metal lead frame, the bond is still often inadequate and failures frequently occur.